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An Electron Released from Rest

  1. Apr 19, 2007 #1
    An electron is released from rest at the negative plate of a parallel plate capacitor. The charge per unit area on each plate is σ = 1.69E-7 C/m2, and the plates are separated by a distance of 1.72E-2 m. How fast is the electron moving just before it reaches the positive plate?

    i started by using the equation E=σ/ϵ. so i got E=1.9096E4N/C. then i multiplied that by one coulomb to get force. so F=3.05536E-15N. i then took newtons seconds law (F=ma) and found acceleration. for the mass of the electron i used 9.109E-31kg. so i was left with a=3.35E15m/s^2. i need to find out velocity, and so i looked for kinematic equations and i cant find one with all the variables that i have. it seems like everyone i try to use is missing 2 variables. am i overlooking something? please point me in the right direction. thanks!
     
  2. jcsd
  3. Apr 19, 2007 #2

    hage567

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    You can use this equation again:

    [tex]v^2 = v_o^2 + 2ax[/tex]

    You have all of the values you need. Which ones are you unsure of?
     
  4. Apr 19, 2007 #3
    I would try to equate difference in PE's with KE and forget kinematics entirely.

    (edit: pretty much end up with Hages eqn either way)
     
  5. Apr 19, 2007 #4

    Dick

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    As long as you've computed the acceleration, if you really want to use kinematics, use d=(1/2)*a*t^2 to compute the time and v=a*t to compute the final velocity. You have a superabundance of choices of how to complete the problem.
     
  6. Apr 19, 2007 #5
    in response to hage567 i dont think that i have the initial velocity. thanks to all three of you.
     
  7. Apr 19, 2007 #6

    Dick

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    "An electron is released from rest". Think again.
     
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